We address the problem of setting up a Hamiltonian theory of gravity when the tetrad (metric) fields are not invertible. We find that this theory exhibits three local degrees of freedom, thus revealing that the limit to a vanishing tetrad determinant is discretely discontinuous. This outcome is shown to be independent of whether the null eigenvalue of the four-metric lies along a timelike or a...
While physically desirable, the extension of Functional Renormalization Group techniques to Lorentzian manifolds is technically challenging and is expected to lead to qualitatively new features. One of these features is the dependence on an underlying vacuum-like state, which I illustrate via Hadamard states in a perturbative expansion. For regulators that render the RHS of the flow equation...
Infrared divergence is a common feature of spinfoam models with a vanishing cosmological constant. The disappearance of divergence at the present of a non-zero cosmological constant was conjectured and then recently proved in a spinfoam model constructed with complex Chern-Simons theory. Based on this spinfoam model, we study how such divergence depends on the value of the cosmological...
The importance of the proper treatment of the wave function renormalization in the renormalization group analysis of quantum gravity is pointed out. The renormalization factor, sometimes called an inessential coupling, can be used to fix any one of the coupling constants, with the exception of the coupling constants that remain unchanged by the rescaling of the field. If we choose to fix the...
In this talk, we will present an alternative paradigm to construct spin foam models that consists in a change in the discretization on which the theory is based. This change produces a classical theory and a corresponding spin foam model with interesting new characteristics. We hope this will help to understand better the rol of the curvature in the quantum domain, to study renormalization of...
The renormalization of composite operators in the Asymptotic Safety scenario is instructive from two points of view, which is due to two different interpretations of their anomalous dimensions. These can be interpreted either as corrections to the actual geometric scaling dimensions of a set of operators, or as approximations of the critical exponents characterizing the Reuter fixed point(s)...
I will present the first direct and non-perturbative computation of the graviton spectral function in quantum gravity. This is achieved with the help of a novel Lorentzian renormalisation group approach, combined with a spectral representation of correlation functions. We find a positive graviton spectral function, showing a massless one-graviton peak and a multi-graviton continuum with an...
The Quantum Geometrodynamics program was an early effort to canonically quantize General Relativity in the most straightforward manner possible. The approach involves a formal $3+1$--split of spacetime and applies Dirac's procedure for the quantization of constraint systems to the ADM formulation of General Relativity, with the configuration variables represented by the components of the...
In the context of perturbative quantum field theory, the addition of quadratic-curvature operators to the Einstein-Hilbert action makes it possible to achieve strict renormalizability in four dimensions. The additional terms are multiplied by dimensionless coefficients that are related to the masses of the extra gravitational degrees of freedom and to the interaction couplings. In this talk,...
Quantum Geometrodynamics represents an early attempt at the canonical quantization of General Relativity. In a seminal paper, DeWitt proposed a formal Hamiltonian constraint operator by substituting canonical momenta with variational derivative operators. However, the rigorous interpretation of this operator has remained elusive due to the highly nonlinear nature of the constraint and the...
Recovering the observed four-dimensional spacetime is a major challenge and distinguishing criterion for any quantum theory of gravity based on fundamentally discrete structures. Tensor models seem to generate random geometries, but only two-dimensional or fractal ones. I will show here how adding geometric degrees of freedom in the spirit of group field theory enables to overcome this...
Spin foam quantum gravity describes quantum space-time as a sum over (pre-)geometric configurations encoded in group theoretic labels. In absence of experimental data, it is therefore crucial to establish connection to classical, continuum physics. While the connection to discrete (Regge) gravity is well established for small discretisations, large triangulations are far less explored. To...
I will present recent work in which we defined a euclidean path integral of gravity and matter fields in the eigenbasis of the wave operators, using a result of Hawking and Gilkey. On one hand, working in the eigenbasis of the wave operators means working exclusively with geometric invariants and this avoids the need to mod out the diffeomorphism group and makes it possible to carry out the...
We obtain the matter-graviton scattering amplitude in the gravitational theory of quadratic curvature, which has R_{μν}^{2} term in the action. Unitarity bound is not satisfied because of the existence of negative norm states, while an analog of unitarity bound for S-matrix unitarity holds due to the cancelation among the positive norm states and negative norm ones in the unitarity summation...
A central question in discrete quantum gravity is how, and in what sense, the continuum can be recovered in the classical limit of the theory. I will address this question in the context of causal set theory, with an emphasis on recent work. In particular, I will discuss how recent developments in Lorentzian geometry can be used to study the convergence properties of causal sets in the large...
We explain how a quantum viewpoint benefits the computation of observables in classical general relativity. We give details on the amplitudes-based approach, highlight recent results and outline challenges faced in pushing the program to higher orders in perturbation theory.
Geometric quantization is a natural way to construct a quantization map over classical field data that is given as a symplectic manifold with an inner product (a Riemannian metic). This yields a (non-commutative) quantum algebra that can be equipped with a state determined by a map dual to the quantization. We investigated this technique for a free scalar field on a causal set (locally finite,...
Interference is among the most universal phenomena in physics, as exemplified by the famous Feynman path integral describing quantum physics as a sum over histories. However, conditionally convergent oscillatory integrals are often delicate to define and even more difficult to evaluate. Moreover, the real-time Feynman path integral is still lacking a mathematically rigorous definition. In this...
It has been shown that Loop Quantum Cosmology (LQC) has the potential to alleviate anomalies related to large scale power suppression, the lensing amplitude and te parity asymmetry present in observations of the CMB. As a consequence of the pre-inflationary dynamics, some modes reach the onset of inflation in an excited state with respect to the Bunch-Davies vacuum, resulting in a scale...
A longstanding issue is the equivalence between the Jordan and the Einstein frames. It is believed, but not completely proved, that the cosmological physical observables give the same results in the two frames. Our aim is to tackle this problem from the perspective of the Hamiltonian formalism. For this reason, we will perform the Hamiltonian analysis of the Brans-Dicke theory with...
The existence of Euclidean wormholes contributing to the Euclidean path integral, and especially their (in)stability, is a long standing puzzle. It has implications both phenomenologically (by breaking the axion global shift symmetry, thus making it a possible dark matter candidate or large-field inflaton, and solving the strong CP-problem of QCD) and from a theoretical perspective, e.g....
The model of Causal Dynamical Triangulations (CDT) is a background-independent and diffeomorphism-invariant approach to quantum gravity, which provides a lattice regularization of the formal gravitational path integral. The framework does not require any coordinate system but uses only geometric invariants. We introduce coordinates via scalar fields with periodic boundary conditions with a...
Studying quantum fields in strong gravity poses several fundamental questions, like the quantum-classical transition of primordial density perturbations in the early Universe and the information paradox. To understand these questions, in this talk, we consider a massive quantum scalar field in a time-dependent background and study various quantum correlation measures — entanglement entropy,...
Quantum Gravity induced corrections at the classical level are usually too small to be relevant for phenomena that can be probed by current observations. In this talk, I will focus on spherically symmetric solutions of Quadratic Gravity. I will discuss a mechanism that ensures that even if corrections to the Schwarzschild solution from General Relativity are small, they become dominant just...
Very compact stars seem to be forbidden in General Relativity. While Buchdahl's theorem sets an upper bound on compactness, further no-go results rely on the existence of two light rings, the inner of which is associated to gravitational instabilities. However, little is known about the role of QFT in these strong gravity regimes. We show that the renormalized stress tensor for CFTs diverges...
I will discuss some aspects of the Everpresent Λ cosmological model arising from fundamental principles in causal set theory and unimodular gravity. In this framework the value of the cosmological constant (Λ) fluctuates, in magnitude and in sign, over cosmic history. At each epoch, Λ stays statistically close to the inverse square root of the spacetime volume. Since the latter is of the order...
Approaches to quantum gravity describing spacetime as emergent, i.e., as the collective behaviour of some discrete fundamental structures, must allow for quantum states which manifest semiclassical features. In this talk I present a realisation of this idea in the context of group field theory (GFT), where we depart from the familiar coherent states by studying properties of the more general...
We will show how to construct the proper low-energy limit of QFT in curved spacetime from a canonically quantized gravity-matter system, discussing four emerging points of previous literature. To address the related shortcomings, we will consider the gravitational sector as composed of a classical background (i.e. a vacuum Bianchi I universe) and quantum fluctuations on top (gravitons)...
We study the implementation of Polymer Quantum Mechanics (PQM) to a system decomposed into a quasi-classical background and a small quantum subsystem, according to the original Vilenkin proposal. We develop the whole formalism in the momentum representation that is the only viable in the continuum limit of the polymer paradigm and we generalize the fundamental equations of the original...
When canonically quantising general relativity, one is faced with the problem of frozen dynamics, also known as the problem of time, which arises due to the presence of constraints in the theory. In this talk, I will provide an overview of the problem of frozen dynamics in general relativity and explain why it is a significant challenge in the field. To study this problem, we have limited...
Quantum information techniques are playing a crucial role in our understanding of quantum aspects of spacetime. In particular for de Sitter space, I will illustrate this through two different examples. Firstly, I will show why momentum-space entanglement is an essential concept in early universe cosmology, as a quantifier for non-unitary evolution, and derive its conceptual ramifications for...
During the career panel, the panelists
Maximilian Becker (Radboud University)
Flaminia Giacomini (ETH Zurich)
Luca Buoninfante (NORDITA)
Kasia Rejzner (University of York)
will share their experiences and insights on applying for jobs in and out of academia, specifically focused on young researchers doing QG. This session will include a Q&A where attendees can ask career related questions.
There are many proposals in the context of loop quantum gravity to effectively describe the main effects predicted by the theory by performing certain specific modifications in the Hamiltonian of general relativity. However, in general, such modifications produce anomalies in the constraint algebra, which imply a loss of covariance of the effective theory. In this work we focus on spherical...
The dawn of gravitational wave astronomy provided multiple channels for probing the physics at work in the very early universe as well as for testing gravity itself. They can be categorised into resolved and unresolved gravitational waves sources, the latter of which embodies a stochastically distribution of signals (similar to the CMB) called the stochastic gravitational wave background...
I will summarise some recent work illustrating a fundamental clash between requiring unitarity in quantum theories of gravity and the notion of general covariance: unitarity means well-defined evolution for arbitrarily long time, but as different notions of time may remain finite or infinite, e.g., on approach to a cosmological singularity, this may or may not require significant departures...
I show how the coincidence limits of the massless, minimally coupled scalar propagator and its first two derivatives facilitate the summation of large logarithms from inflationary gravitons using a variant of Starobinsky's stochastic formalism. I derive analytic approximations for these correlators that are valid for any cosmology which has undergone an epoch of primordial inflation. A...
Black hole thermodynamics lies at the crossroads of quantum gravity, information theory, and relativistic field theory, and provides fundamental clues about the microscopic degrees of freedom of gravity. In this talk, I will discuss the insights we have gained from our attempts to understand black holes as thermodynamic systems. I describe the phase structure of black hole spacetimes, and the...
When the gravitational field is treated quantum-mechanically, the classical trajectories of falling objects are subject to random fluctuations ("noise"). This fundamental noise might be observable at gravitational wave detectors and, if detected, would provide experimental evidence for the quantization of gravity. The noise also modifies the Raychaudhuri equation, thereby enhancing the...
A complete theory of quantum gravity is generally expected to cure the singularities inherent to the mathematical black holes predicted by general relativity. In the absence of such a theory, singularity-free models of so-called regular black holes have become a popular alternative to avoid the nontrivial causal structures typically associated with mathematical black holes. In this talk, we...
We introduce an approach to compute the renormalization group flow of relational observables in quantum gravity which evolve from their microscopic expressions towards the full quantum expectation value. This can be achieved by using the composite operator formalism of the functional renormalization group. These methods can be applied to a large class of relational observables for different...
In this talk, I will show that In gravity, the subregion entanglement is controlled by a symmetry group called the corner symmetry group, which follows from invariance under diffeomorphism of the total space. This universal symmetry group gives us semi-classical phase space tools to understand quantum geometry in the continuum. It also constrains the entanglement structure of subregions. I...
Understanding the fundamental nature of gravity at the interface with quantum theory is a major open question in theoretical physics. Recently, the study of gravitating quantum systems, namely quantum systems sourcing a gravitational field and interacting gravitationally, has attracted a lot of attention, thanks to the possibility of realising these scenarios in the laboratory in the near...
Traditionally observational signatures of quantum gravity (QG) have been considered far out of reach of observation. We take another look at this question, and discuss how UV/IR mixing, in theoretically controlled contexts (such as fluid gravity near horizons and shockwave backgrounds), could give rise to the appearance of the IR scale in QG effects in experiments.
We consider the quantum gravity partition function that counts the dimension of the Hilbert space of a spatial region with topology of a ball and fixed proper volume, and evaluate it in the leading order saddle point approximation. The result is the exponential of the Bekenstein-Hawking entropy associated with the area of the saddle ball boundary, and is reliable within effective field theory...
Quantum spin networks are a phenomenal quantitative tool to investigate the operational content of the gravitational field at the quantum scale. In this light, spin network entanglement has recently become a central resource to characterize dynamical signatures and holographic behaviour of quantum spacetime geometry, providing a new tool to study the emergence of classical spacetime geometry...
In appropriate semiclassical limits, the so-called island formula computes the entropy of non-gravitational quantum systems entangled with a gravitational theory. This is a special case in which the quantum-corrected Ryu-Takayanagi formula has been shown to compute a von Neumann entropy using only properties of the gravitational path integral and, in particular, without relying on the...
Tensor models provide an interesting direction in the pursuit of quantum gravity. Understanding their dynamics would be an important part of the pursuit, which could be partly captured by distributions of tensor eigenvalues in tensor models, similarly to Wigner's semi-circle law in matrix models. We show how the distributions can be computed by field theoretical tools, and discuss some...
We revisit the problem of time in the context of quantum cosmology. By considering more than one scalar matter field propagating on an FLRW cosmology, we identify several Dirac observables from which two evolving constants of motion (ECM) are constructed. One of these ECMs will play the role of the clock and against which the other ECM is measured. We quantize both of these ECMs which amounts...
We explore two hypotheses. First, the possibility that the quantum vacuum energy density of the Casimir effect contributes to a (local) gravitational vacuum energy density. Second, the possibility that a change in the gravitational coupling implies a change in the cosmological constant. We parametrize these two possibilities in a covariant framework and show that the next generation of Casimir...
In 2005 an article with a similar title by Manrique et al presented an implementation of Wilson’s renormalization group and continuum limit tailored to loop quantization together with a nontrivial example leading to an interacting relativistic quantum field theory. I will review that proposal, compare with other proposals, give new examples and critically examine the application of this...
A model of an extended manifold for the Dirac spinor field is considered. Two Lagrangians related by CPTM (charge-parity-time-mass) symmetry are constructed for a pair of the Dirac spinor fields with each spinor field defined in a separate manifold. An interaction between the matter fields in the manifolds is introduced through gravity. A fermionic effective action of the general system is...
The Barrett-Crane (BC) spin foam and GFT model is a state-sum model which provides a tentative quantization of first order Lorentzian Palatini gravity written as a constrained BF-theory. It is conjectured that this model gives rise to continuum spacetime with General Relativity as an effective description for the dynamics at criticality via phase transition. In this talk, we discuss how phase...
I report on recent progress in the computation of gauge-invariant relational observables around highly symmetric backgrounds, to arbitrary orders in perturbative quantum gravity and without introducing extra fields which change the dynamics. I then explain how one can compute quantum gravitational corrections to the Newtonian gravitational potential of a point particle using these observables....
Extracting the physics of cosmological inhomogeneities and anisotropies from full quantum gravity is a crucial step to make contact with observations. I address this problem within the group field theory (GFT) formalism for quantum gravity by studying the perturbative mean-field effective dynamics of small relational inhomogeneities of GFT condensates. I show how these perturbations give rise...
There are not many tools to quantitatively monitor the emergence of classical geometric features from a quantum spacetime, whose microscopic structure may be a highly quantum-fluctuating “spacetime foam”. To improve this situation,we introduce new quantum observables that allow us to measure the absolute and relative homogeneity and isotropy of geometric properties of a nonperturbative quantum...
Quantum mechanics allows for states in macroscopic superpositions, but they ordinarily undergo rapid decoherence due to interactions with their environment. A system that only interacts gravitationally, such as an arrangement of dark matter (DM), may exhibit slow decoherence. In this work, we compute the decoherence rate of a quantum object within general relativity; a robust and rigorous...
One of the possible applications of quantum computers in the near future are quantum simulations of physical systems. In Loop Quantum Gravity the quantum geometry of space is represented by superposition of the so-called spin networks. A construction of quantum circuits that generate states of the Ising-type spin networks is described. The results of the implementation of the approach on the...
I study the balance law equation of surface charges in the presence of background fields. The construction allows a unified description of Noether's theorem for both global and local symmetries. From the balance law associated with some of these symmetries, I will discuss generalizations of Wald's Noether entropy formula and general entropy balance laws on null hypersurfaces based on the null...
